# How to fit a sine curve with only the maximum and minimum values

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Patricia Hauser il 8 Apr 2024
Commentato: Alexander il 9 Apr 2024
Hi, I am trying to plot a sine wave through the high and low water points from a measuring station in the North Sea.
The image shows the first few data points. I've tried fitting a curve, filling the gaps etc. but I can't get Matlab to acknowledge the oscillating signal.
Help will be appreciated, thank you!
this is the goal:
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Mann Baidi il 8 Apr 2024
Hi,
What type of 'fitType' are you passing in the 'fit' function?
Sam Chak il 8 Apr 2024
It might be easier to fit a custom sinusoidal waveform if the data is converted to the unit of 'seconds'. However, it is important to note that the time step in the t vector is not uniform.
where and are the time-varying amplitude and the time-varying period of the sine wave, respectively.
T = 58x2 table
t_datetime t_datenum ____________________ _________ 01-Jan-2022 03:05:00 -1.3 01-Jan-2022 09:02:00 1.5 01-Jan-2022 15:32:00 -1.4 01-Jan-2022 21:34:00 1.4 02-Jan-2022 04:08:00 -1.4 02-Jan-2022 10:06:00 1.5 02-Jan-2022 16:31:00 -1.4 02-Jan-2022 22:29:00 1.5 03-Jan-2022 05:03:00 -1.6 03-Jan-2022 11:04:00 1.5 03-Jan-2022 17:24:00 -1.4 03-Jan-2022 23:20:00 1.6 04-Jan-2022 05:56:00 -1.6 04-Jan-2022 11:59:00 1.5 04-Jan-2022 18:16:00 -1.4 05-Jan-2022 00:09:00 1.7
x = T{:,1}; % datetime data
t = seconds(x - x(1)); % convert to seconds
A = T{:,2}; % high and low points of the sea level
stem(t, A), grid on, xlabel('Time (sec)'), ylabel('Sea level')
title('Sea level around the North Sea from 1st to 15th, Jan 2022')

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### Risposte (2)

Bruno Luong il 8 Apr 2024
Modificato: Bruno Luong il 8 Apr 2024
piecewise (co)sine interpolation (not fitting).
I don't think you have extra DOF to play with if you restrict data to be local extrema and model follows a sinusoidal law.
t=datenum(data.t_datetime);
dt = t-t(1);
y = data.t_datenum;
dti = linspace(min(dt),max(dt),1025);
yi = sininterp(dt, y, dti); % function defined below
% Ploy check result
figure
subplot(2,1,1)
dtfun = @(dt) datetime(dt+t(1),'ConvertFrom','datenum');
h = plot(dtfun(dt),y,"ob",dtfun(dti),yi,"r");
% zoom plot
ax2 = subplot(2,1,2);
h = plot(dtfun(dt),y,"ob",dtfun(dti),yi,"r");
xlim(ax2, dtfun([8 11]))
%%
function yi = sininterp(t, y, ti)
ti = ti(:);
i = discretize(ti, t);
yi = nan(size(ti));
valid = i>=1 & i<numel(ti);
i = i(valid);
y1 = y(i);
y2 = y(i+1);
ym = (y1+y2)/2;
A = (y1-y2)/2;
alpha = (ti(valid) - t(i)) .* (pi./(t(i+1)-t(i)));
yi(valid) = ym + A.*cos(alpha);
end
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Alexander il 9 Apr 2024
Perfect. I'll put it in my stock for special purposes.

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R il 8 Apr 2024
Hi Particia,
Fitting a piecewise cubic Hermite ("pchip") curve might help with this use case. Here is an example of the same
% Read data from Excel file
% Convert first column to string array for time labels
time = string(table2array(data(:, 1)));
% Extract water level data from second column
waterLevel = table2array(data(:, 2));
% Create index array for plotting
x=(1:58)'; % Ensure x is a column vector
% Plot water level data
plot(x,waterLevel,".");
title('Water Level vs. Time');
xticklabels(time) % Set custom x-axis tick labels
% Fit a piecewise cubic Hermite interpolating polynomial
[curve,gof,output] = fit(x,waterLevel,"pchipinterp");
% Overlay fitted curve on the plot
hold on
plot(curve)
hold off
Here is the plot I get after executing the above code:
You can also find a list of curve fitting models that MATLAB supports in the following documentation:
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